1. Field of the Invention
This invention relates to temperature responsive control devices and more particularly to the design and construction of a hydraulic diaphragm for controlling the operation of the electrical switching mechanism of a thermostat for use in a domestic oven.
2. Description of the Prior Art
Hydraulic type thermostatic control devices, having an elongated bulb or probe formed at the end of a capillary tube that is connected to a bellows or diaphragm within the thermostat housing, have been widely used as oven thermostats. This general type of thermostat is disclosed in the Ettinger U.S. Pat. No. 2,260,014 and the Staples U.S. Pat. No. 3,293,394 both of which are assigned to the General Electric Company, the assignee of the present invention. Prior art bellows or diaphragms generally comprised two shallow cup-like members arranged in nested or telescoping relation and hermetically sealed together at their outer edges. One end of the capillary tube was connected to the bellows or diaphragm by a machine type stud. At the other end of the capillary tube was a temperature-sensing bulb or probe, also well known in the art. The diaphragm, capillary tube and temperature-sensing bulb formed a closed system which, when filled with a suitable thermally responsive fluid, produced mechanical movement in the diaphragm in response to the expansion and contraction of the fluid as a function of the oven temperature change sensed by the bulb. Mounted on the top of the diaphragm stud was a circular pin the top of which was freely received within a mating hole in the lower end of a shaft which formed part of a manually-adjustable mechanism. The bottom of the diaphragm was seated upon a snap-acting spring mechanism which in turn controlled an electrical switching means. Setting a desired temperature by the manually adjustable mechanism had the effect of moving the diaphragm with respect to the snap-acting spring mechanism. A small clip mounted on the bottom of the diaphragm engaged the spring mechanism to prevent the diaphragm from rotating with the shaft.
In operation, as the temperature inside the oven increased, the fluid within the closed system expanded, thereby causing the diaphragm to expand and put pressure upon the snap-acting mechanism. Upon reaching the desired temperature, the diaphragm pressure caused to snap-acting mechanism to move over center thereby opening the switch. As the temperature inside the over decreased, the fluid inside the closed system contracted thereby reducing the pressure of the diaphragm upon the snap-acting spring mechanism and closing the switch.
Although prior art thermostats were generally adequate for controlling and maintaining oven temperatures, there was still a need for a simpler and more reliable device. The nested cup-type diaphragms were expensive to manufacture and assemble. The machine type stud which was used to attach the capillary tube and to be internally bored in order to allow for the passage of fluid, a machining operation which introduces significant expense. This resulted in additional cost and manufacturing complexity. The use of the small clip to prevent rotation of the diaphragm also resulted in increased cost and assembly problems. In addition, the flat or semispherical top of the stud, which was employed in the prior art to mate with a surface in the bottom of the shaft, tended to wear unevenly, thereby resulting in a reduction in the thermostat reliability and accuracy. This type of stud also presented assembly problems in that due to its shape, it was often difficult to get it to properly mate with the bottom of the shaft.
An object of the present invention is to provide a temperature responsive control device having a simplified diaphragm which provides for increased ease in assembly, more precise temperature calibration and improved accuracy.
A further object of the present invention is to provide a temperature responsive control device having a capillary tube which is connected directly to the diaphragm in a simple and inexpensive manner.
A further object of the present invention is to provide a temperature responsive control device with a simplified diaphragm which prevents relative rotational movement between the diaphragm and the housing in a simpler manner.
A further object of the present invention is to provide a temperature responsive control device having greater contact area between the diaphragm stud and connecting control shaft, thereby resulting in more precise adjustments, less mechanical wear and improved reliability.